1. Field of the Invention
The invention relates to a hydraulic control apparatus for an automatic transmission. More particularly, the invention relates to a hydraulic control apparatus for an automatic transmission, which can achieve a shift speed even when an electric failure has occurred.
2. Description of the Related Art
A hydraulic control apparatus for an automatic transmission has been known which can achieve an intended shift speed thereby allowing a vehicle to run, even when an electric failure has occurred and an automatic transmission cannot be electrically controlled.
Japanese Patent Application Publication No. JP-A-2001-248718 discloses a control apparatus for an automatic transmission, which can achieve a shift speed even when an electric failure has occurred. The control apparatus for an automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-2001-248718 includes a C1 hydraulic servo which controls a C1 clutch; a normal open type C1 solenoid valve which controls supply/drain of hydraulic pressure to/from the C1 hydraulic servo; a C2 hydraulic servo which control a C2 clutch; a normal open type C2 solenoid valve which controls supply/drain of hydraulic pressure to/from the C2 hydraulic servo; a C2B2 supply relay valve which controls supply/drain of hydraulic pressure to/from the C2 solenoid valve; a C3 hydraulic servo which controls a C3 clutch; a normal open type C3 solenoid valve which controls supply/drain of hydraulic pressure to/from the C3 hydraulic servo; a B1 hydraulic servo which controls a B1 clutch; a normal open type B1 solenoid valve which controls supply/drain of hydraulic pressure to/from the B1 hydraulic servo; and a B1 release valve which is provided between the B1 hydraulic servo and the B1 solenoid valve, which achieves drain communication with the B1 hydraulic servo using the hydraulic pressure supplied from the C3 solenoid valve, and which interrupts the hydraulic pressure supplied from the B1 solenoid valve.
With this control apparatus for an automatic transmission, when the C1 solenoid valve is de-energized and the other three solenoid valves are energized at normal time, hydraulic pressure is supplied only to the C1 hydraulic servo and the C1 clutch is engaged, whereby first speed is achieved. When an electric failure has occurred, in the case where one of first to fourth speeds is achieved or in the case where the hydraulic pressure in a D range oil passage is drained and then hydraulic pressure is re-supplied to the D range oil passage, hydraulic pressure is supplied to the C1 hydraulic servo through the C1 solenoid valve, whereby the C1 clutch is engaged. When an electric failure has occurred, the C2B2 supply relay valve interrupts the hydraulic pressure from an oil pump to the C2 solenoid valve. Accordingly, the C2 clutch is disengaged. Hydraulic pressure is supplied to the C3 hydraulic servo through the C3 solenoid valve, and the C3 clutch is engaged. Due to the hydraulic pressure from the C3 solenoid valve, the B1 release valve achieves drain communication with the B1 hydraulic servo, and the hydraulic pressure from the B1 solenoid valve is interrupted. Accordingly, a B1 brake is released. Namely, when an electric failure has occurred, third speed can be achieved. Since third speed can be achieved when an electric failure has occurred, the vehicle is allowed to run at third speed.
According to the invention disclosed in Japanese Patent Application Publication No. JP-A-2001-248718, in order to achieve a shift speed when an electric failure has occurred, normal open type solenoid valves are used as the four solenoid valves which control supply/drain of hydraulic pressure to/from friction engaging elements. Accordingly, unless the three solenoid valves from among the four solenoid valves are energized, first speed cannot be achieved. Therefore, electric power consumption is increased. The electric power to be supplied to the solenoid valves is usually generated by an alternator coupled with a crankshaft of an engine. Accordingly, as the electric power consumption is increased, a load placed on an engine is also increased, which causes a problem that the fuel efficiency is reduced.